An antiserum raised against the synthetic tyrosinylated carboxy-terminal sequence of synenkephalin (Tyr-Glu-Glu-Ser-His-Leu-Leu-Ala) has been used to chromatographically characterize the human synenkephalin-like immunoreactivity extracted from 3 adrenal medullary phaeochromocytomas. Gel filtration chromatography identified in each tumor a single peak of 8 kDa which on subsequent ion-exchange chromatography had the elution characteristics of an acidic polypeptide. These results are compatible with the human synenkephalin sequence predicted from cDNA studies, and indicate that this is the authentic peptide.